It is well recognized that the safe disposal of municipal and industrial waste products is an extremely important issue that is now receiving a great deal of attention. The issue is not only to develop adequate techniques and means of handling these wastes but to also correct the serious problems that been created in the past due to improper and unsafe disposal practices. Serious contamination of the environment has resulted from the lack of knowledge, or intent, to implement cost effective methods for the disposal, storage and containment of these undesirable products. The problem is magnified many fold by the great number of waste disposal and mineral waste areas.
In many situations such as landfills and mineral mining operations, where on line or in-situ processing is technically not feasible or other reasons make it cost prohibitive, a reasonable alternative is to store, contain and create an isolation covering over those wastes, provided this can be accomplished successfully. This course of action has been pursued at many locations with some success but with all too many failures.
In efforts to develop effective covers for waste, the unique characteristics of water-swelling clays have been used extensively. The mineral sodium montmorillonite, generally known as sodium Bentonite, has been one of these commonly used clays. As reported in an article authored by S. Karaborni et al. in the Feb. 23, 1996 issue of the publication Science, "c!lays have a characteristic layered structure. Between these layers, water can adsorb, which results in strong repulsive forces that cause the clays to expand to as much as several times their original thickness. Clay hydration studies have been conducted since 1933, yet there is no clear understanding of the swelling mechanism." Nevertheless, water-swelling clays have been used by industry in a large number of products and in a variety of applications.
The use of hydrated clay solutions in drilling operations of the oil and gas industry is a good example of how the unique expanding property of Bentonite clay is used to advantage. The drilling fluid, commonly known as "mud," performs a number of functions. In brief, the mud is pumped down the drilling shaft where its circulation lubricates and cools the drill bit, helps seal the bored formations, prevents pressure blowouts and most importantly holds drill cutting particles in suspension while the particles are carried away and brought to the surface. U.S. Pat No. 5,426,079 to Hughes describes in detail the characteristics of water-swellable clays, particularly Bentonite, and their use in drilling muds.
U.S. Pat. No. 5,439,317 to Bishop et al. discloses a process of suspending solid particles in a hydrated clay solution. A first step involves the creation of a hydrated clay solution. Several other steps require the addition of solid particles of mining by-products to the hydrated solution for the purpose of suspending a large quantity of particles in the solution while transporting them to an underground storage location.
Some soil sealing applications mix Bentonite clays with other ingredients in both a wet and dry condition. For example, U.S. Pat. No. 3,016,713 to Deming discloses a method of treating soil surfaces with an aqueous slurry of a lattice clay (Bentonite) and an additive of a water-soluble anionic polyelectrolyte to render the walls and bottoms of stock ponds and other water reservoirs impervious. U.S. Pat. No. 3,986,365 to Hughes teaches a soil sealing method by adding an amount of a particular water-soluble polymer to a dry Bentonite-soil admixture whereby the polymer addition decreases the amount of Bentonite necessary to form a water containing soil enclosure. U.S. Pat. No. 3,772,893 to Eilers teaches a soil sealing method to reduce the permeability of soil solids to water by mixing a dry mixture of an expanding lattice clay and a linear water soluble organic polymer to a layer of soil. The a final step of the process involves compacting the treated layer of soil.
Bentonite clays also serve as water barrier elements in geosynthetic fabric liners. Those liners consist of a layer of dry Bentonite clay sandwiched between two nonwoven geotextile fabrics. U.S. Pat. No. 5,346,566 to White and U.S. Pat. No. 5,112,665 to Alexander each describe geosynthetic fabric liner systems. Geosynthetic clay liners of this type serve as landfill covers and water barriers to eliminate the need of a compacted clay layer. Although those liners have low hydraulic conductivity specifications and are cost competitive, when compared with compacted clay, seam integrity is a potential problem and material and installation costs remain a concern.
Compacted clay covers are a traditional method of complying with the requirements of the U.S. Environmental Protection Agency. Compacted clay typically comprises a bottom layer of a three layer system, and can consist of native soil mixed with a 4-8% by weight of dry Bentonite clay. Compacting this layer slightly "wet of optimum" initially achieves the design hydraulic conductivity; however, those designs have been shown to be vulnerable to desiccation cracking, especially when installed in arid environments. Desiccation cracking drastically reduces the effectiveness of the cover and can even result in cap failure. In general, compacted clay covers not only have inherent problems but are very expensive and difficult to construct.
Because of the inadequacies of the current state-of-the-art environmental protection measures, various governmental agencies are actively engaged in programs which promote the development or improvement of environmental restoration and management technologies. A key and integral part of this program is an effort to encourage the development and implementation of new and innovative landfill cover design and installation procedures.